The Tor Browser / file revision

 /* This Source Code Form is subject to the terms of the Mozilla Public

  * License, v. 2.0. If a copy of the MPL was not distributed with this

  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

 'use strict';

 module.metadata = {

   'stability': 'unstable'

 };

 const { Cc, Ci, Cu } = require('chrome');

 const AppShellService = Cc['@mozilla.org/appshell/appShellService;1'].

                         getService(Ci.nsIAppShellService);

 const NS = 'http://www.w3.org/1999/xhtml';

 const COLOR = 'rgb(255,255,255)';

 /**

  * Creates canvas element with a thumbnail of the passed window.

  * @param {Window} window

  * @returns {Element}

  */

 function getThumbnailCanvasForWindow(window) {

   let aspectRatio = 0.5625; // 16:9

   let thumbnail = AppShellService.hiddenDOMWindow.document

                     .createElementNS(NS, 'canvas');

   thumbnail.mozOpaque = true;

   thumbnail.width = Math.ceil(window.screen.availWidth / 5.75);

   thumbnail.height = Math.round(thumbnail.width * aspectRatio);

   let ctx = thumbnail.getContext('2d');

   let snippetWidth = window.innerWidth * .6;

   let scale = thumbnail.width / snippetWidth;

   ctx.scale(scale, scale);

   ctx.drawWindow(window, window.scrollX, window.scrollY, snippetWidth,

                 snippetWidth * aspectRatio, COLOR);

   return thumbnail;

 }

 exports.getThumbnailCanvasForWindow = getThumbnailCanvasForWindow;

 /**

  * Creates Base64 encoded data URI of the thumbnail for the passed window.

  * @param {Window} window

  * @returns {String}

  */

 exports.getThumbnailURIForWindow = function getThumbnailURIForWindow(window) {

   return getThumbnailCanvasForWindow(window).toDataURL()

 };